OBJECTIVE To determine the effects of estrogen on the pathway from the nucleus retroambiguus (NRA) to the lumbosacral cord. To determine the effects of estrogen on the pathway from the nucleus retroambiguus (NRA) to the lumbosacral cord, in six ovariectomized female rhesus monkeys wheat-germ agglutinin horseradish peroxidase (WGA-HRP) injections were made into the NRA. Three of the animals were estrogen treated, the remaining three served as controls. The density of the anterograde projection was determined in the motoneuronal cell groups that have previously been shown to receive distinct projections from the NRA axial, abdominal and pelvic floor. At the ultrastructural level, WGA-HRP-labeled terminal profiles in these motoneuronal cell groups were analyzed and growth cones were identified. The results showed that the density of the NRA-lumbosacral motoneuronal projection in estrous monkeys is slightly higher than in controls. At the ultrastructural level, WGA-HRP-labeled growth cones were found in estrous monkeys. The presence of growth cones in the NRA-lumbosacral pathway in estrous females strongly sug gests that estrogen induces axonal outgrowth in this pathway. FUTURE DIRECTIONS The differences in the density of the NRA-lumbosacral projection need to be quantified, and the tissue of non-estrogen treated ovariectomized females needs to be further analyzed at the ultrastructural level. KEY WORDS nucleus retroambiguus, lumbosacral cord, anterograde tracing, estrogen, growth cone, sprouting FUNDING Supplemental funding to RR00167